1. Field of the Invention
This invention relates generally to optical mark readers, and pertains more particularly to a reader or scanning system in which automatic provision is made for opacity differences in the document paper and also in the ink used in printing the document.
2. Description of the Prior Art
In the past it has been necessary to manually adjust or calibrate each photoelectric cell of an optical mark scanner once the paper and ink characteristics were determined for a particular batch of answer documents. This has involved the resetting or "tweeking" of the potentiometer for each of the photoelectric cells. Even so, should the opacity of the document paper vary within a given batch, then the calibration would be incorrect for those sheets deviating from the sheet on which the adjustment was based. The same thing holds true for variations in the opacity of the ink used in printing the various documents or forms.
It can be pointed out that the most common change in ink opacity between batches of answer documents would be in the color of the ink. For instance, red ink barely affects the ink opacity above that of the paper as "seen" by the photocells, whereas blue or green has an appreciable effect. Purple and brown change the opacity to a lesser degree than blue or green but still have a greater influence than red. Also, in the past, no ready check has been available as far as determining whether each photocell is providing a proper response and has become bad. Because of this, a number of answer documents could be incorrectly scored or tabulated before a bad photocell is detected.